1. Field of the Invention
The present invention relates to an optical disk having a plurality of recording layers, and a recording method of an optical disk having a plurality of recording layers.
2. Background Art
FIG. 2 schematically shows a cross-sectional structure of a conventional multilayered optical disk, and a principle of selectively reading/writing information from/to each of the recording layers. In the present prior art form, the recording medium includes a total of five recording layers (the first recording layer 411, the second recording layer 412, the third recording layer 413, the fourth recording layer 414, and the fifth recording layer 415). When the five-layer recording medium is used, in order to access recorded information on the second recording layer 412, the position of an optical spot 32 is positioned on the second recording layer 412 by controlling the position of an objective lens 30. In this case, convergent light 31 during the course of being focused by the objective lens is transmitted through the first semitransparent recording layer 411. However, the beam diameter of the convergent light 31 on the first recording layer is sufficiently large as compared with the diameter of the optical spot 32 on the second recording layer 412, and hence the recorded information on the first semitransparent recording layer 411 cannot be decomposed and reproduced. The beam diameter is large on the first semitransparent recording layer 411, and hence the light intensity per unit area is relatively small. Therefore, there is no possibility that the information on the first recording layer 411 is destroyed at the time of recording. In this way, reading/writing of information from/to the second recording layer formed farther than the first recording layer is realized without being influenced by the first recording layer.
Similarly, when reading/writing of information from/to the fifth recording layer 415 is performed, the position of the optical spot 32 is positioned on the fifth recording layer 415 by controlling the position of the objective lens 30. Here, in the case where the interlayer spacing is set as d, where the numerical aperture of the objective lens is set as NA, where the wavelength of light is set as λ, and where the refractive index of the interlayer transparent layer is set as n, the beam diameter on the layer adjacent to the reading/writing target layer is given by 2×d×(NA/n)/(1−(NA/n)2)1/2. For example, in the case where d is 8 μm, and where NA is 0.85, the beam diameter becomes about 10 μm. Thus, in the case where the wavelength λ is 400 nm, the light beam on the adjacent layer has a diameter more than 20 times as large as the diameter (λ/NA=470 nm) of the optical spot 32 on the targeting layer, and hence has an area more than 400 times as large as the area of the optical spot 32. The details of the conditions, under which reading/writing is performed in this way from/to an optical recording medium having a plurality of recording layers without the influence of the other layers, are described in Patent Publication (Kokai) No. 5-101398 (patent document 1).
In such a multilayered optical disk, in the case where information is recorded on a farther layer as viewed from the light incident side, there is a problem that the laser power reaching the farther layer is different due to the difference in the effective transmittance of the nearer layer between the case where the information is recorded on the farther layer through an unrecorded area on the nearer layer and the case where the information is recorded on the farther layer through a recorded area on the nearer layer. This problem is schematically shown in FIG. 5. FIG. 5A shows a state where an optical spot is focused on the n-th recording layer (the n-th layer). FIG. 5B shows a state of an optical beam transmitted through a recording layer (the m-th layer) on the nearer side than the n-th layer in the case where the optical spot is focused on the n-th layer. The vertical lines in the figure represent recording tracks formed on the m-th layer. An area 431 means an area (unrecorded area) where no information is recorded, while an area 432 means an area (recorded area) where certain information is recorded. In the case of a Blu-ray Disc, the track pitch is 0.32 μm, and hence the light made incident on the m-th layer is transmitted through the m-th layer by being spread in a range including about 100 tracks, which also depends on the distance from the n-th layer. The transmittance is different between the recorded area and the unrecorded area. Thus, even in the case where the same optical beam is transmitted, the amount of transmitted light is different between the case where the optical beam is transmitted through the recorded area and the case where the optical beam is transmitted through the unrecorded area. That is, the effective transmittance of the m-th layer is changed by the area ratio between the recorded area and the unrecorded area which exist in the m-th layer.
In JP Patent Publication (Kokai) No. 2003-109217 (patent document 2), in order to cope with this problem, a recording medium is configured such that the difference in the transmittance between the unrecorded portion and the recorded portion of the nearer layer is set to a constant value or less, so that recording can be performed on the farther layer by constant recording power independently of the recording states of the nearer layer.
As described in JP Patent Publication (Kokai) No. 2003-109217, in the case where the optical design of the nearer side layer (the m-th layer) is performed, it is preferred to perform the design so that the transmittance is not changed between the unrecorded area and the recorded area. However, the transmittance difference of several % to about 10% between the unrecorded area 431 and the recorded area 432 is normally caused due to various factors including variation in manufacture of the medium, a design error, and the like. Further, even when the transmittance of the nearer layer can be made the same between the unrecorded area and the recorded area, the reflectance is different between the unrecorded area and the recorded area, and hence the reproduced signal quality of the farther layer may be changed by the influence of the reflected beam from the nearer layer.
Therefore, in an actual medium, a slight difference exists in the transmittance between the unrecorded area 431 and the recorded area 432. Thus, as shown in FIG. 5, the laser power reaching the farther side n-th layer is different due to the difference in the effective transmittance of the m-th layer between the case where the recording is performed on the farther side n-th layer through the unrecorded area 431 of the nearer side m-th layer, and the case where the recording is performed on the farther side n-th layer through the recorded area 432 of the nearer side m-th layer. More precisely, when an optical spot 321 is focused on the farther side n-th layer, the effective transmittance of the nearer side m-th layer is not changed in a binary manner in dependence upon whether or not the nearer side m-th layer is recorded, but is continuously changed in correspondence with the area ratio between the unrecorded portion and the recorded portion of an optical beam 322 on the nearer side m-th layer.
There will be described below an example of the influence of this phenomenon on the recording condition learning. FIG. 3 shows relationships between recording power and jitter at the time when reading/writing is performed from/to a farther side L0 layer in a two-layer recordable type optical disk in respective cases where the nearer side L1 layer is not recorded and where the nearer side L1 layer is recorded. A limit equalizer normally used for a Blu-ray Disc is applied as a signal processing method at the time of evaluating the reproduced signal, and the signal is represented as the magnitude of the signal jitter value. According to the measurement, it was found that when all the area of the nearer side L1 layer is not recorded, the optimum recording power, that is, the recording power at which the jitter is minimized is 7.1 mW, and the jitter at this time is 6.7%. On the other hand, it was also found that when all the area of the nearer side L1 layer is recorded, the optimum recording power is 7.5 mW. That is, the optimum recording power in the case where the L1 layer is recorded is shifted by about 7% to the higher power side as compared with the case where the L1 layer is not recorded. If in the case where the L1 layer is recorded, the recording is performed by using the recording power of 7.1 mW which is the optimum power in the case where the L1 layer is not recorded, the jitter becomes 7.0%, and hence is increased by 0.3% as compared with the case where the optimum recording power is used.
This result has, for example, the following meaning. It is assumed that when recording condition learning is performed for the L0 layer, the portion on the L1 layer, through which portion the laser beam is transmitted, is not recorded, and that the optimum recording power determined in this state is used to perform recording on the entire L0 layer. Then, the recording can be performed with no problem in the case where the portion on the L1 layer, through which portion the laser beam is transmitted, is not recorded. However, in the case where the portion on the L1 layer, through which portion the laser beam is transmitted, is recorded, the jitter of the signal reproduced from data recorded on the L0 layer is increased. That is, the margin of the effective recording power is reduced. Therefore, in the method described in patent document 2, the margin of the effective recording power is reduced, and hence it is difficult to perform the recording with high reliability on the entire farther layer by using constant recording power.
In the prior art forms which are proposed to avoid this problem in patent document 3 (JP Patent Publication (Kokai) No. 2005-038584) and patent document 4 (JP Patent Publication (Kokai) No. 2004-327038), multilayer areas for optimum power control are configured so as not to overlap each other so that the power control is always performed in the state where the nearer side layer is not recorded. Further, in patent document 5 (JP Patent Publication (Kokai) No. 2008-192258), the problem of error in the learning of recording power is avoided by a method in which the recording power is learned in both the case where the other layer is recorded, and the case where the other layer is not recorded, and in which the results of the learning are averaged.
Patent document 1: JP Patent Publication (Kokai) No. 5-101398 (U.S. Pat. No. 5,414,451)
Patent document 2: JP Patent Publication (Kokai) No. 2003-109217
Patent document 3: JP Patent Publication (Kokai) No. 2005-038584 (U.S. Patent Application Publication No. 2004/0264339)
Patent document 4: JP Patent Publication (Kokai) No. 2004-327038
Patent document 5: JP Patent Publication (Kokai) No. 2008-192258